Differential method of photocurrent measurement

ABSTRACT

A differential amplifier circuit for measuring radiation induced photocurrents in small geometry semiconductor devices.

United States Patent 1191 Stehlin et a].

[ Dec. 25, 1973 DIFFERENTIAL METHOD OF PHOTOCURRENT MEASUREMENT [75]Inventors: Robert A. Stehlin; Hilton W.

Spence; Walter T. Matzen, all of Richardson, Tex.

[73] Assignee: The United States of America as represented by theSecretary of the Air Force, Washington, DC

[22] Filed: Nov. 16, 1971 21 Appl. No.: 199,211

[52] US. Cl. 324/123 R, 324/158 D [51] Int. Cl. G01r 1/30, GOlr 31/26[58] Field of Search 324/123, 121, 158 D,

[56] References Cited UNITED STATES PATENTS 3,464,773 9 1969 Waz 356 2153,518,438 6 1970 Hart et al 356/223 3,528,350 9 1970 Schmitt 356/2223,664,752 5 1972 Hermieu 356/224 Primary ExaminerRudolph V. RolinecAssistant ExaminerErnest F. Karlsen AttorneyI-larry A. Herbert, Jr.

[5 7 ABSTRACT A differential amplifier circuit for measuring radiationinduced photocurrents in small geometry semiconductor devices.

2 Claims, 6 Drawing Figures DIFFERENTIAL METHOD OF PHOTOCURRENTMEASUREMENT BACKGROUND OF THE INVENTION When an electronic deviceutilizing semiconductors is operated in an environment which issubjected to radiation such as gamma rays, hole electron pairs aregenerated within the semiconductor material. These carriers move bydiffusion and drift to and through the semiconductor junctions. Theeffect of these carriers passing through the semiconductor junction isto produce transient photocurrents. The current components which enterthe base region are called the primary photocurrents. The majorcomponent of primary photocurrent is produced in the collector regionand in the transition region of the collector-base junction. The emittercomponent of the primary photocurrent is, in general, substantiallysmaller than the major component of the primary photocurrent. The shortdiffusion length which is utilized in the emitter contributes to thesmaller emitter component of the primary photocurrent.

In the prior art, the measurement of photocurrents in semiconductorcircuits was at best very difficult and subject to gross discrepancies.This is due to the fact that the instruments or devices which areutilized to make the photocurrent measurements are themselves subjectedto and affected by the radiation conditions that produce thephotocurrents in the semiconductor devices. The present inventionprovides a method of photocurrent measurement which are simple andaccurate. The adverse affect's'which are associated with the operationof electronic equipment in a radiation environment have been eliminatedin the present measurement technique.

SUMMARY OF THE INVENTION The present invention utilizes a differentialamplifier to measure the photocurrents which are generated in a testdevice that is operated in a radiation environment or subjected to someform of radiation. The differential amplifier test circuit is calibratedin a simulated radiation environment without the test device connectedto the. differential test circuit. The calibration establishes a baseline against which the photocurrent measurement will be compared. Thetest device is connected to the differential amplifier test circuit andthe response of the combination is recorded. The difference between thetwo responses represents the photocurrents which are generated in thetest device.

It is one object of the invention, therefore, to provide an improvedphotocurrent measurement apparatus utilizing a differential amplifiercircuit to provide an accurate and reliable photocurrent measurement.

It is another object of the invention to provide an improvedphotocurrent measurement apparatus having the capability of measuringphotocurrents in a test device in a simulated radiation environment.

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in conjunctionwith the illustrative embodiment in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of thedifferential amplifier photocurrent measurement circuit in accordancewith the invention;

FIG. 2 is a schematic diagram of the differential amplifier which isutilized in the photocurrent measurement circuit;

FIG. 3 is a graphical plot of test results of photocurrents measured ina test device versus radiation dose rate;

FIG. 4a is a block diagram of the basic photocurrent measurementcircuit;

FIG. 4b is a block diagram of a charge scattering measurement circuit;and

FIG. 40 is a block diagram of an air ionization potential measurementcircuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG.1, there is shown a photocurrent measurement circuit utilizing adifferential amplifier 10. The differential amplifier 10 has first andsecond input terminals, 11 and 12. The first input terminal 11 isconnected to a 50 ohm terminating resistor 13 and the device under test14 (diode). The diode 14 is connected to a 5 volt power supply voltagewhich is applied at terminal 15. The second input terminal 12 isconnected to a 50 ohm terminating resistor 16. The test device 14 isconnected to the first input terminal 11 during the measurement of thephotocurrents which are generated within the test device. Thedifferential amplifier 10 has a pair of output terminals, l7, 18. Theoutput signals of the differential amplifier 10 appear on outputterminals 17, 18 and are coupled, respectively, to amplifiers 19, 20 bycapacitors 21, 22. The input terminal 23 of amplifier 19 has aterminating resistor 25 connected therebetween to ground. A terminatingresistor 26 is also provided between input terminal 24 of amplifier 20to ground. The output signals from amplifiers 19, 20 are applied throughcoax cables 27, 28 to a measurement apparatus. The measurement apparatuswhich is shown in the present embodiment is an oscilloscope 29. Theoscilloscope 29 may be any commercially available apparatus having a 50MHtz response, such as a Tektronixs, model 547 with a 1A5 difierentialcomparator amplifier plug in unit. The Tektronix scope is manufacturedby Tektronix Inc, Beaverton, Oregon, 97005. The oscilloscope 29 has apair of terminals 30, 31 to which coax cables 27, 28 are respectivelyconnected. The terminals 30 and 31 are labeled A and B on the Tektroxixplug-in unit, model 1A5 (shown in FIG. 1). Any signals applied toterminals A and B are differenced and the result shown on the scopescreen. Neither terminal A or B is referenced to ground. The output endof coax cable 27 is terminated with a 50 ohm terminating resistor 33 toground. The output end of coax cable 28 is likewise terminated with a 50ohm terminating 32 to ground.

The photocurrent measurement of a test device is accomplished in thefollowing manner. The present photocurrent measurement depends uponcalibrating the differential amplifier circuit which is shown in FIG. 1without connecting the test device to the input terminal 11. Thedifferential amplifier 10 has no amplifier input leads which may beconnected to the header or other system wiring. The device to be testedis built on the same chip. The differential amplifier I0 is completelyfree of any unnecessary external connections which may createinaccuracies or errors in the photocurrent measurement. The differentialamplifier 10 test circuit while operating without a test deviceconnected is calibrated in a simulated radiation environment. Thepositive 5 volt voltage which is applied to terminal is supplied todifferential amplifier 10 to provide a base line on oscilloscope 29.Since there is no test device connected to input terminal 11 ofdifferential amplifier 10, the base line established by the positive 5volt voltage may be superimposed upon the photo-current response whichappears on the oscilloscope 29. Thus, after the initial calibration ofthe differential amplifier 10 test circuit in the simulated radiationenvironment, the test device 14 is connected to the differentialamplifier 10 with a small (x 2 mils) internal jumper. The modifieddifferential amplifier test circuit with the test device connected tothe input terminal 11 is again exposed to the simulated radiationenvironment. The photocurrents which are generated within the testdevice are applied through the differential amplifier 10, amplifiers 19,and coax line 27 and 28 to terminal 30 and 31 of oscilloscope 29. Thebase line which established the photo-currents within the test circuitwithout the test device connected, now provides a reference point fromwhich the photocurrents which are generated within the test device maybe measured. The difference between the two responses is related to thephotocurrent of the test device by the following formula:

I A V /R X Gain; where AV is the change in the voltage out, R is theresistor which is connected from the test device input terminal toground, and Gain is the gain of the differential amplifier.

Turning now to FIG. 2, there is shown a schematic diagram of thedifferential amplifier which was utilized in the photocurrentmeasurement test circuit of FIG. 1. The circuit is comprised of twotransistors 40, 41 which have their emitters respectively connected andare operated in the differential amplifier circuit configuration. Aconstant current source 42 is connected to the common emitter point 43between the two transistors 40, 41. The constant current source 42comprises a transistor 44 having its collector connected to the commonemitter point 43 and its emitter connected through a resistor 45 to thenegative input voltage supply terminal 46. The base of transistor 44 isconnected to the negative input voltage supply terminal 46 by a parallelcircuit comprising diode 47 and resistor 48 in parallel with diode 49.Diodes 47 and 49 are connected in opposite polarity with respect to eachother between the base of transistor 44 and the negative input voltagesupply terminal 46. The output signals from transistors 40, 41 are takenfrom the respective collectors and connected to the respective bases oftransistors 50, 51. The transistors 50, 51 are operated in the emitterfollower circuit configuration. The output signals from the transistors50, 51 appear at the respective emitters of transistors 50, 51 and areapplied to output terminals 52, 53 respectively. The collectors oftransistors 40, 41, 50, 51 are connected through resistors 54, 55 and 56respectively to the positive input voltage supply terminal 58.

' The results which have been obtained by utilizing the differentialamplifier test circuit (shown in FIG. 1) to measure photocurrents in atest device are shown in FIG. 3. The photo-current, l,,,, is shownplotted against the gamma dose rate, 'y The absolute value agreessubstantially with the theory and a linear slope is obtained. Thedifferential amplifier has been illustrated in an embodiment by whichphotocurrents may be accurately measured. The differential amplifier maybe utilized in other circuit configurations to measure other radiationparameters. A few of these are shown in FIGS. 4a through 40. FIG. 4aillustrates the basis block of the differential amplifier test circuitwhich was shown in detail in FIG. 1. FIG. 4b illustrates how thedifferential amplifier may be arranged to receive an input signal from ametallic conductor 60. This circuit configuration provides a chargescattering measurement circuit. The charge scatter current is given bythe formula:

I A Vout/R X Gain FIG. 4c illustrates a circuit configuration utilizingthe differential amplifier to provide an air ionization potentialmeasurement circuit. A pair of metallic conductors, 62, 64 arerespectively connected to the first and second input terminals 66, 68respectively, of the differential amplifier. The air ionizationpotential is given by the formula:

V A Vout/Gain.

Although the invention has been described with reference to a particularembodiment, it will be understood to those skilled in the art that theinvention is capable of a variety of alternative embodiments within thespirit and scope of the appended claims.

We claim:

1. A differential amplifier test circuit for measuring the photocurrentswhich are generated within a test device operating in a simulatedradiation environment comprising in combination:

' a differential amplifier having a first and second input terminal anda first and second output terminal, said first input terminal beingconnected through the device to be tested to a positive power supplyvoltage terminal, said first input terminal having a terminatingresistor connected therebetween to ground, said second input terminalhaving a terminating resistor connected therebetween to ground,

a first amplifier being coupled to said first output terminal of saiddifferential amplifier, said first amplifier having an input and anoutput terminal, said input terminal having a terminating resistorconnected therebetween to ground, said output terminal being connectedto one end of a 50 ohm coax cable, said coax cable having a shield, saidshield being grounded at the amplifier end,

a second amplifier being coupled to said second output terminal of saiddifferential amplifier, said second amplifier having an input and anoutput terminal,

said input terminal having a terminating resistor connected therebetweento ground, said output terminal being connected to one end of a 50 ohmcoax cable,

said coax cable having a shield, said shield being grounded at theamplifier end, and,

an oscilloscope having a first and second input terminal, said firstinput terminal being connected to the other end of said first amplifiercoax cable, said first input terminal having a terminating resistorconnected therebetween to ground, said second input terminal beingconnected to the other end of said second amplifier coax cable, saidsecond input terminal having a terminating resistor connectedtherebetween to ground.

2. A differential amplifier test circuit as described in claim 1 whereinsaid differential amplifier comprises:

a first and second transistor having their emitters connected, saidfirst transistor base being said first input terminal, said secondtransistor base being said second input terminal,

a constant current source connected to said emitters of said first andsecond transistors, said constant current source being connected to anegative power supply voltage terminal,

a first emitter follower transistor having its base connected to thecollector of said first transistor, said first emitter followertransistor having an emitter, said emitter having a terminating resistorconpower supply voltage terminal.

1. A differential amplifier test circuit for measuring the photocurrentswhich are generated within a test device operating in a simulatedradiation environment comprising in combination: a differentialamplifier having a first and second input terminal and a first andsecond output terminal, said first input terminal being connectedthrough the device to be tested to a positive power supply voltageterminal, said first input terminal having a terminating resistorconnected therebetween to ground, said second input terminal having aterminating resistor connected therebetween to ground, a first amplifierbeing coupled to said first output terminal of said differentialamplifier, said first amplifier having an input and an output terminal,said input terminal having a terminating resistor connected therebetweento ground, said output terminal being connected to one end of a 50 ohmcoax cable, said coax cable having a shield, said shield being groundedat the amplifier end, a second amplifier being coupled to said secondoutput terminal of said differential amplifier, said second amplifierhaving an input and an output terminal, said input terminal having aterminating resistor connected therebetween to ground, said outputterminal being connected to one end of a 50 ohm coax cable, said coaxcable having a shield, said shield being grounded at the amplifier end,and, an oscilloscope having a first and second input terminal, saidfirst input terminal being connected to the other end of said firstamplifier coax cable, said first input terminal having a terminatingresistor connected therebetween to ground, said second input terminalbeing connected to the other end of said second amplifier coax cable,said second input terminal having a terminating resistor connectedtherebetween to ground.
 2. A differential amplifier test circuit asdescribed in claim 1 wherein said differential amplifier comprises: afirst and second transistor having their emitters connected, said firsttransistor base being said first input terminal, said second transistorbase being said second input terminal, a constant current sourceconnected to said emitters of said first and second transistors, saidconstant current source being connected to a negative power supplyvoltage terminal, a first emitter follower transistor having its baseconnected to the collector of said first transistor, said first emitterfollower transistor having an emitter, said emitter having a terminatingresistor connected therebetween to ground, said emitter being said firstoutput terminal, and, a second emitter follower transistor connected tothe collector of said second transistor, said second emitter followertransistor having an emitter, said emitter having a terminating resistorconnected therebetween to ground, said emitter being said second outputterminal, said collectors of said first and second transistors beingconnected to a positive power supply voltage terminal, said first andsecond emitter follower transistors having their respective collectorsconnected to said positive power supply voltage terminal.